Particle size is a significant parameter for particles, which can be used to characterize the behavior of an aerosol. Particle sizes need to be measured in many situations. For example, particles formed in the semiconductor industry are critical for microcontamination control. Particles formed in chemical reactors are of great interest in material science. To obtain the size information about such particles, differential mobility analyzers are commonly selected as a tool.
Differential mobility analyzers (DMAs) have been widely applied in a variety of aerosol studies and applications, especially for particles in the submicron and nanometer diameter ranges. The DMA method of size classification is based on the electrical mobility of a singly charged particle being inversely related to the size of the particle. A polydisburse aerosol containing singly charged particles over a range of sizes can be classified according to size in an electric field and produces a nearly monodisburse aerosol within a narrow range of electrical mobilities. Thus, the produced aerosol contains particles of substantially the same size. The primary functions of DMAs are for particle sizing and classification. As a sizing instrument, scanning mobility particle analyzers (SMPS) combine a DMA with an ultrafine condensation particle counter (UCPC). For the classification, the evaporation-condensation-DMA classification method has been utilized to generate monodisperse particles. DMAs have also been used to generate PSL standard particles for instrument calibration by removing impurity peak. For bio-application, DMAs with high resolution have been utilized to analyze proteins.
The most widely used DMA is the one commercialized by TSI Inc, St. Paul (TSI Model 3701). Although this DMA works well in the size range of 20 to 500 nm, it becomes increasingly difficult to perform accurate measurement/classification for particles less than 10 nm. The sizing resolution and detection sensitivity of such DMAs deteriorate in the single digit nanometer size range due to Brownian motion of the particles. Meanwhile, nanometer particles have received significant attention because of their special electrical, optical, and/or magnetic properties, making them suitable for high-tech applications. Therefore, it is necessary to develop an optimized DMA able to adequately study aerosols below the 10 nm diameter range.
The diffusion loss inside DMAs increases with smaller particles. The resolution of DMAs worsens with decreasing particles size due to the effect of particle diffusion. To minimize the effect, DMAs with short columns are used to measure particles of single digit nanometer sizes. The largest particle size measured is determined by the maximum electrical strength achievable in a specific DMA. DMAs of long column lengths are needed for larger particles.
Alternative DMAs have been designed for wide particle size ranges. One is an adjustable-column length DMA (ACLDMA) disclosed by Seol et al, 2002 (See Seol, K. S., Yabumoto, J., Takeuchi, K. (2002), “A Differential Mobility Analyzer with Adjustable Column Length for Wide Particle-Size-Range Measurements”, J. Aerosol Sci., 33: 1481–1492). By a servomotor and a reducing gear, the classification length of ACLDMA can be adjusted between 0 to 300 mm. ACLDMA can measure a wide particle size range from 1 nm to 500 nm. ACLDMA overcomes the limitation of the traditional DMA of constant classification column. ACLDMA, however, can only classify one particle size at a time as the conventional DMA. For a bi-model distribution aerosol, where the two peaks are far away from each other, ACLDMA cannot measure the two peaks simultaneously. Furthermore, mechanical wear can cause the concern of DMA axial alignment and gas leak. ACLDMA is also much bigger than the conventional DMA
Another DMA technique, electric aerosol spectrometer, has also been designed to measure aerosols with wide size range using multi-electrodes. This device has two commercial versions: Differential Mobility Spectrometer (DMS) and TSI 3090 Engine Exhaust Particle Sizer (EEPS), which have been developed to cover the nucleation and accumulation modes in an internal combustion engine exhaust. DMS is able to measure particles in the size range from 5 nm to 1000 nm with 26 electrons. By using electrometer as particle detector, the response time is of 500 ms. Similarly, EEPS is capable of measuring aerosol in the 5.6 nm to 560 nm size range with 22 electrons. The response time of EEPS is 0.1 ms. Both DMS and EEPS are suitable to study transient events in the engine operation.
DMS and EEPS do not have adequate resolution because the entire size range is divided into only 32 channels. Moreover, EEPS can only distinguish two peaks that differ by a factor of 3 in size. Furthermore, there is current leak between the adjacent electrodes, which can interfere with the measurement of particle size distribution. The current leak makes it impossible to measure liquid particles. Therefore, dry air is used to remove any liquid coatings from the solid particles. Due to the detection limit of the electrometer, DMS and EEPS can only measure high concentration aerosol. Since the electrodes are located inside of the classification region, the backgrounds of the electrodes keeping changing with the voltage applied on inner rod. Therefore DMS and EEPS cannot be used to scan particle size distribution in the same way as DMA. Since no sampling flow is drawn out of the instruments, it is impossible to determine their transfer function experimentally. The transfer function is simply obtained by running a model over a range of particle sizes.